Hold-type display devices such as liquid crystal display devices have a problem that, in a case where a moving image is displayed, a blurred image (so-called motion blur) is observed by a viewer since there occurs a gap between a point of viewer's gaze following an object and a position of the object that is being displayed.
One example of a method for solving the motion blur problem is a method of providing a display period of a minimum luminance level (e.g., black display of 0% luminance level) in a part of 1 frame period.
However, this method has a problem that flicker is likely to occur since bright and dark states are repeated on an entire screen in each frame. Moreover, there is a problem that, in a case where a luminance level of a video signal is maximum (e.g., white display of 100% luminance level), providing a display period of a minimum luminance level within 1 frame period causes a reduction in luminance level.
In view of this, as a technique for reducing motion blur without causing flicker, a technique (so-called frame interpolation technique) is proposed in which an interpolation frame is generated and inserted within 1 frame period (Patent Literature 1 etc.).
The following describes the frame interpolation technique. FIG. 19 illustrates an image of a first frame and an image of a second frame, and illustrates a state in which a black object displayed on a left side in the first frame moves to a right side in the second frame. An interpolation frame is generated based on a motion vector calculated from a video signal of the first frame and a video signal of the second frame. For example, in a case where a frame rate of the video signal is increased 4 times (quad-speed drive), a motion vector which corresponds to a travel distance in the first frame and the second frame is divided into quarters, and three interpolation frames are generated based on the motion vectors thus obtained.
FIG. 20 illustrates a state in which the three interpolation frames are inserted within first and second frame periods. As illustrated in FIG. 20, the three interpolation frames in which the black object gradually moves to the right side with time are inserted in the first and second frame periods. This makes motion of the black object smooth, thereby reducing motion blur.
In the frame interpolation technique, a search range for a travel distance of an object (motion vector) is generally set within 200 ppf (pixels per field) in 1 frame period in consideration of power consumption and driving load. Only in a case where a motion vector is found within this range, an interpolation frame in which a position of the object is estimated based on the motion vector is generated (first generation mode).
Meanwhile, no interpolation frame corresponding to the motion vector is generated in (i) a case where the travel distance of the object exceeds the search range, (ii) a case where the travel distance of the object falls in the search range, but an object displayed in a first frame and an object displayed in a second frame are not recognized as an identical object due to a minor change in brightness and color, or (iii) a case where a video signal is not a signal of a moving image (e.g., video in which a still image displayed in a first frame is lost in a second frame). In such cases, an interpolation frame having an intermediate gradation which is obtained by distributing gradations of video signals in previous and subsequent frames according to a time component is generated (second generation mode). This is intended to preserve luminance balance and color balance as compared with a smooth motion of the object attained by the first generation mode.
As described above, in the frame interpolation technique, a range in which an interpolation frame corresponding to a motion vector can be generated (first generation mode) is limited. Accordingly, there are cases where a motion vector is not calculated even in a case where video signals are of a moving image of an identical object. In such cases, an interpolation frame having an intermediate gradation is generated (second generation mode).
FIG. 21 illustrates a state in which interpolation frames each having an intermediate gradation are inserted. As illustrated in FIG. 21, a black object displayed on a left side in a first frame gradually changes from black to white with time, and lost in a second frame. Meanwhile, no object is present on a right side of a display frame in the first frame, but color gradually changes from white to black with time, and a black object appears in the second frame. In a case where a motion vector is not calculated, interpolation frame of intermediate gradations are thus inserted.
By the way, currently, a MVA (multi domain vertical alignment mode) mode and an IPS (In-Plane Switching) mode are mainly adopted as liquid crystal modes of liquid crystal display devices from the viewpoint of contrast and viewing angle characteristic. However, a response time in these liquid crystal modes is approximately 4 msec in the fastest gradation region, approximately 8 msec in average, and as slow as 15 msec in the slowest gradation region.
Accordingly, in liquid crystal display devices driven at a frequency (120 Hz) double the video signal of 60 Hz, overshooting (tone transition emphasis process) is generally used. The overshooting allows an improvement in response speed of liquid crystals, thereby improving moving image response characteristics.